Ascorbic acid is an essential nutrient in human nutrition often referred to as its common name, vitamin C. Vitamin C is an important water soluble antioxidant in humans, and in this role it prevents the oxidation of cell and tissue components by free radicals. Vitamin C nutrition in humans in interesting because humans do not have the ability to synthesise vitamin C from glucose like most other mammals, and so a dietary source is required. Vitamin C is found primarily in foods of plant origin and fruits and vegetables are rich sources of the vitamin. Vitamin C is concentrated in the chloroplasts of plants where it might function in the photosynthetic process. The human diet can likely satisfy the metabolic needs of man for vitamin C if it is of high quality and rich in fruits and vegetables. However work by Linus Pauling and others have clearly shown that supplementation with pharmacological doses of vitamin C may have further benefits, particularly with regard protection from cancer and cardiovascular disease.
High intakes of vitamin C are not therefore uncommon in those who choose to benefit from its pharmacological benefits. Indeed supplements of vitamin C are not generally found in less that around 500 mg tablets, a dose that would provide close to the equivalent amount of vitamin C as even the highest quality plant rich diet. Of course the cheap price of vitamin C and its efficacious nature at preventing disease has attracted the propagandists from Big pharma who do not wish to see their profits affected by healthy people. In this regard all manner of accusations have been made regarding the safety of high intakes of vitamin C in order to persuade the public that they should avoid supplementation. In particular one myth that is often peddled is that high intakes of vitamin C can cause the formation of calcium oxalate crystals that can then accumulate in the kidney leading to the formation of kidney stones. However, safety studies do not support the contention that vitamin C is able to increase the risk of kidney stones in healthy human subjects.
One study that investigated the effects of vitamin C on calcium oxalate metabolism administered 10 gram intakes of ascorbic acid and measured the urinary excretion of oxalate1. The vitamin C was provided in five 2 gram doses, which is likely the upper amount that can be tolerated by most individuals. The results showed that mean urinary oxalate excretion was enhanced from about 50 mg to 87 mg per day. When the vitamin C supplements were discontinued the mean urinary oxalate excretion returned to baseline levels within a day. Detailed analysis of the oxalate excretion revealed that after 6 grams of ascorbic acid, a plateau of oxalate excretion occurred and further intakes of vitamin C up to 10 grams had no further effects on excretion. The increases in oxalate excretion in this study were low and these values would not cause problems in healthy individuals. High doses of vitamin C therefore appears to show no detrimental effect on oxalate excretion in the urine, suggesting that vitamin C at high intakes does not cause kidney stones.
Dr Robert Barrington’s Nutritional Recommendation: Linus Pauling the two time Nobel prize winning biochemist recommended grams amounts of vitamin C per day based on his research. At this pharmacological intake he suggested that vitamin C was able to improve immunity and decrease the risk of developing atherosclerotic plaques. Kinetic studies investigating vitamin C have shown that plasma levels can increase up to doses of around 2.5 grams per day in health individuals. Under conditions of stress, plasma levels of vitamin C drop and during such times even higher intakes may be beneficial. It is recommended that individual consume between 500 mg and 3 grams of vitamin C per day. This figure can be increased to between 2 and 5 grams under times of stress, and up to 10 grams in cases of severe illness. A high quality diet rich in plant foods can provide roughly 600 mg per day. Interestingly the authors in this study also measured the urinary excretion of calcium and found that ascorbic acid was able to reduces its urinary excretion, suggesting that vitamin C affects calcium balance positively.
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